The present invention relates to an image reading apparatus of the type using CCDs (Charge Coupled Devices) or similar reading elements and, more particularly, to an image reading apparatus desirably applicable to an electronic copier, facsimile transceiver or similar image recording apparatus.
Generally, an image reading apparatus reads an image carried on, for example, an original document by CCDs or similar reading elements via optics while moving the document and the reading elements relative to each other. The reading elements photoelectrically convert the image to produce an image signal. The image signal is written to a storage as image data, displayed on a display, and/or transferred to a printer to be printed out on a recording medium. A controller has a CPU (Central Processing Unit) and a clock generating circuit or a timing generating circuit which generates a clock signal for clocking the reading elements. The problem with a conventional image reading apparatus is that once the power source of the apparatus is turned on, all the circuits including the CCDs are rendered operative, i.e., they are constantly driven even when actual image reading operations are not needed. The CCDs therefore remain in a stand-by state over a longer period of time than in an image reading state, not only aggravating power consumption but also accelerating temperature elevation inside the apparatus.
An image reading apparatus elaborated to eliminate the above problem is disclosed in Japanese Patent Application No. 213082/1989. Specifically, the apparatus disclosed in this Patent Application has a timing generating circuit for feeding clock signals such as a transfer clock and a shift clock to CCDs or similar image reading elements, a gate for selectively interrupting the delivery of the clock signals from the timing generating circuit to the CCDs, and a controller for enabling the gate in the event of image reading. This kind of implementation, however, brings about another problem since some components of a processing circuit included in the apparatus have particular time constants. Specifically, if a timing signal is fed to such a processing circuit simultaneously with the start of an image reading operation, the processing circuit has to process an image signal before it fully starts up, rendering the image signal defective. For example, it takes about several hundred milliseconds for CCDs to fully start up. Hence, should CCDs be driven simultaneously with the start of image reading, several hundred lines of data would be outputted in an unstable condition. On the other hand, regarding circuit components without any time constant, it is preferable to delay the generation of a timing signal from the electromagnetic interference and power consumption standpoint.